1. Field of the Invention
The present invention relates to a discharge tube for voltage control and, more specifically, to a gas-filled discharge tube for operation as the series gap of the ignition system of an automotive spark-ignition engine.
2. Description of the Prior Art
The ignition system of an automotive spark-ignition engine applies a high voltage across the electrodes of a spark plug to pass an electrical discharge between the electrodes. A previously proposed, ignition system of a series gap type employs a discharge gap connected in series to a spark plug to control the ignition timing accurately and prevent the smoking of the explosive mixture. Such an ignition system of a series gap type employs a gas-filled discharge tube comprising a tube filled with an inert gas, and electrodes provided respectively at the opposite ends of the tube. The accurate control of the spark timing by using the gas-filled discharge tube serving as a series gap requires a discharge inception voltage of the discharge tube higher than that of the spark plug, for example, the former not lower than 10 KV. The discharge inception voltage of the tube can be increased by increasing the distance between the electrodes or by increasing the pressure of the inert gas filling the tube. However, such a measure for increasing the discharge inception voltage entails unstable discharge inception voltage and requires a comparatively high discharge sustaining voltage, which increases energy loss and affects adversely to the reliability of the igniting function of the spark plug.